The present invention relates generally to field plate transistors, and more particularly to field plate transistors with increased breakdown voltage and reduced capacitance between the gate and drain.
Very generally speaking, a conventional transistor design has a source, a drain, and gate. The output current of the transistor is controlled by the voltage applied to the gate. However, in many conventional transistor designs, the breakdown voltage is lower than desired, and a capacitance forms between the gate and the drain. This capacitance reduces the gain of the transistor.
The two main sources of a transistor's gate-to-drain capacitance are an inter-electrode capacitance (between the gate metalization and the drain metalization) and a capacitive coupling between the gate and drain (due to the space charge region in the semiconductor material). The space charge region in the semiconductor material extends from a point beneath the gate electrode to the drain of the transistor.
One attempt to reduce this capacitance has been to place a conductor between the gate and drain. The conductor is electrically isolated from the substrate of the transistor by a dielectric, and electrically connected to the source. See, e.g., U.S. Pat. Nos. 5,119,149, 5,252,848, and 6,091,110, all of which are incorporated herein by reference. This conductor is typically called a “field plate.” The field plate increases the breakdown voltage of the transistor by redistributing the electric field at the gate edge of the transistor such that the gate-drain voltage is dropped across the dielectric layer instead of the semiconductor surface.
In practice, the field plate is positioned so close to the gate that self-aligned techniques are needed to obtain the desired spacing. Materials used in self-aligned processes, such as Titanium Tungsten Nitride, are highly resistive; and, because the field plate is elongated across the transistor, the voltage drops along the field plate. Thus, the field plate's ability to reduce capacitance is diminished in the regions away from the field plate connection to the source. As used herein, the term “highly resistive” refers to materials which, in the application of field plates in transistors, have resistivities sufficient to cause a change of potential along the length of a field plate that results in a significant change in performance (e.g., Titanium Tungsten Nitride and Tungsten Silicide). Such materials are in contrast to examples of less resistive materials (e.g., gold and aluminum).